Background Radiation Sources

Introduction

When a Geiger counter is operated it will usually record an event every few seconds, even if no obvious radioactive source is placed nearby. Where do these counts come from?

Two significant sources are cosmic rays and radon decay products. Cosmic rays, as the name suggests, are fast-moving particles from space that enter the Earth’s atmosphere, along with their decay products. Since the atmosphere absorbs some of these particles, the rate of detection of cosmic rays increases with increasing altitude. If you were to take a Geiger counter on a cross-country jet flight, you would observe a marked increase in count rate while at high altitude.